In large electrical machines with a hydrogen-cooled rotor, the internal space of the machine casing must be hermetically sealed against the external conditions. This requires comprehensive arrangements both at the sealing points between the outer casing and the end casing and in the region of the shaft penetration. In addition, the end casing must be so designed that it withstands the internal pressure attained of up to 10 bar. The article "Turbogeneratoren (turbo-generators)", published in the "ETG-Fachberichte 3, Kraftwerks-Generatoren" VDE-Verlag GmbH, Berlin, 1977, pp. 7-32, provides a survey, which is still up to date, of the design of the casing/end casing of turbo-generators with gas cooled rotor windings. The one-piece (FIG. 23a on Page 29) or horizontally split (FIG. 19 on Page 28) end casing has a multiplicity of radially extending reinforcement ribs. The sealing between the outer casing and the end casing is, in fact, simpler in the case of one-piece end casings compared with horizontally split casings. Since, however, the shaft seal can only be fitted after the final installation of the rotor, the bearing blocks must be at a sufficient distance from the end casing for the shaft seal to be fitted. The increase in the length of the machine associated with this is, however, undesirable.
A further problem is presented by the access to the end parts of the stator. This access is at best provided -- if at all -- in the known machines by means of manholes in the outer casing, which increase the length of the machine, but not in the strongly ribbed end casings.